A steam generator (hereinafter, referred to as “SG”) that generates steam by boiling secondary cooling fluid with the heat of primary cooling fluid is provided with a plurality of heat-transfer tubes and a tube plate to which ends of the heat-transfer tubes are welded, thereby supporting the heat-transfer tubes. The secondary cooling fluid outside the heat-transfer tubes is heated by the primary cooling fluid that flows inside the heat-transfer tubes. The tube plate not only supports the heat-transfer tubes but also constitutes a partition between the primary cooling fluid and the secondary cooling fluid.
Accordingly, welded portions (hereinafter, referred to as “heat-transfer-tube sealing portions”) between the heat-transfer tubes and the tube plate are required to prevent leakage or the like of the primary cooling fluid to the secondary-cooling-fluid side.
In this case, if there is a defect in the heat-transfer-tube sealing portion, the defect may form an opening toward the primary-cooling-fluid side or the secondary-cooling-fluid side due to pressure, etc. from the primary cooling fluid, and a pathway (leakage pathway) through which the primary cooling fluid or the like leaks may be formed.
Therefore, in the final stage of SG manufacturing, integrity verification is performed in which an SG is subjected to a pressure-endurance test to confirm the presence/absence of leakage pathways. However, it is preferable that defects at the heat-transfer-tube sealing portions be detected in advance before performing such a test.
In the case in which such a defect is a crack that opens at a surface of the heat-transfer-tube sealing portion, the defect can be detected by a permeation test (PT) or a leakage test. However, there is a problem in that an internal defect such as a blowhole, that is, a defect which does not open at the surface, cannot be detected by such inspection methods.
On the other hand, a radiation test (RT) can also detect internal defects in the heat-transfer-tube sealing portions. With the RT, however, the defect inspection speed is lower than in the above-described inspection methods. Accordingly, inspecting all of heat-transfer-tube sealing portions in an SG having a few thousand heat-transfer tubes with RT is exceedingly time consuming, and thus, it is not practical.
Although a blowhole can be detected by RT inspection, there is a problem in that the depth (distance from the surface to the blowhole) of the blowhole cannot be determined.
Accordingly, it is desirable to apply the eddy-current flaw detection method (ECT) with which relatively rapid inspection can be performed for defects at the surfaces of the heat-transfer-tube sealing portions, and various inspection methods have been proposed (for example, see Patent Literatures 1 and 2).